A solder mask is a hard, permanent layer of non-conductive material which covers the surface of a printed circuit board or the like, encapsulating the traces of the printed circuit itself. By solder mask is meant herein a hard, permanent layer which meets the abrasion resistance tests as defined in IPC-SM-840A, Table 12, Summary of Criteria for Qualification/Conformance (Institute for Interconnecting and Packaging Electronic Circuits). The solder mask is patterned to fully cover the circuitry except for those portions intended to be exposed, e.g., for soldering to another component. Solder masks are typically formed from a layer of photoimageable composition which is applied to a surface of the printed circuit board. The photoimageable layer is exposed to actinic radiation which is patterned by means of a template or artwork. Subsequent to exposure, the photoimageable layer is developed in an organic solvent or an aqueous solution which washes away either exposed or unexposed portions of the layer (depending upon whether the photoimageable material is positive acting or negative acting). The portion of the layer which remains on the surface is then cured, e.g. with heat and/or UV light to form a hard, permanent solder mask intended to protect the printed circuitry for the life of the board.
One prior art method of applying the layer of photoimageable composition to the circuit board surface is to apply the material in liquid form and then either allow it to dry or partially cure the material to form a semi-stable layer.
There are a number of advantages to applying a photoimageable layer to a circuit board as a dry film rather than as a liquid. In particular, dry films are free of organic solvent and therefore eliminate this hazard from the workplace and eliminate the need for apparatus to protect the immediate work environment and the more general environment from organic solvent emissions. Typically, a dry film comprises a cover sheet of support material which is somewhat flexible but which has sufficient rigidity to provide structure to a layer of photoimageable composition which overlies one surface of the support sheet. Typically, the cover sheet is formed of polyester material, such as polyethylene terephthalate (PET), such as that sold as MELINEX.RTM..
To protect the photoimageable layer and to enable the dry film to be rolled, it is conventional that the exposed surface of the photoimageable layer be covered with a removable protective sheet e.g., a sheet of polyethylene. An example of such a dry film is sold as LAMINAR DM.RTM. by the Dynachem Division of Morton Thiokol, Inc.
The method of use of such a prior art dry film is generally as follows. The protective sheet is removed from the photoimageable composition layer immediately prior to application of the dry film to the surface of the printed circuit board. This may be accomplished, for example, using automated apparatus which peels away and rolls up the protective sheet as the dry film is unrolled from a reel. The dry film is applied to the surface of the circuit board with the photoimageable layer in direct contact with the board surface. Using heat and vacuum, the photoimageable layer is immediately laminated to the surface of the board. The cover sheet remains overlying the photoimageable layer, protecting the photoimageable layer from exposure to oxygen and from handling damage. The cover sheet also permits an artwork (or template) to be laid directly on top of the dry film for contact printing, if contact printing is to be used (as is usually preferred from the standpoint of obtaining optimal image resolution). The dry film is exposed to patterned actinic radiation through the PET cover sheet. At this time, the PET support sheet is removed, permitting access to the exposed photoimageable layer by developer. Depending upon the composition of the photoimageable layer, the photoimageable layer is developed with organic solvent, aqueous developer, or semi-aqueous developer, i.e., a developer including both water and organic solvent. The photoimageable layer may either be positive acting, in which case the exposed portions are removed by developer, or negative acting, in which case the unexposed portions are removed by developer. Most photoimageable layers for preparing solder masks are negative acting. Most photoimageable composition layers require some cure subsequent to development to render the layer hard and permanent so as to serve as a solder mask. Depending upon the composition of the photoimageable layer, curing may be effected with heat and/or UV light.
Although a number of advantages accrue through the use of dry films as opposed to coating the circuit board with a liquid composition, there are some problems with solder mask-forming dry films which the present invention addresses.
Because the cover sheet of a conventional dry film is relatively rigid, the photoimageable layer cannot fully conform to the irregular contours of the printed circuit board, from which circuitry traces rise from the flat surface of the board itself. Because of this, the thickness of the photoimageable layer must be slightly greater than the elevation of the traces above the surface of the board. For example, if the traces rise 75.mu. above the surface of the board, the photoimageable layer will typically be about 100.mu. thick; when laminated to the board, approximately 25.mu. of photoimageable composition overlies the upper surfaces of the traces.
One unfortunate result of the solder mask being thicker than the elevation of the traces is that elevated regions of photomask exist between those portions of the circuitry from which photoimageable composition is removed during the development of the solder mask, e.g. those portions of the traces to which a component is to be soldered. Elevated regions are particularly problematic with respect to components which are surface-mounted to the printed circuit board. Components typically contain a plurality of pins which are each to be soldered to an exposed portion of a trace of the circuit board. If the component does not fit closely to the board, it may rest on the surface of photoimageable composition without adequate bonding being established between the pins and the traces. It would be desirable if low regions, or "valleys", rather than elevated regions existed between exposed trace portions.
Some common traces are narrower at the foot than at the top, in which case small overhangs extend outward along each side of each trace (an effect known as "mushrooming"). With both liquid compositions and conventional dry films, there inherently remains air-filled voids below such overhangs. The air trapped in such voids may produce some oxidation of the traces and may lead to eventual defects in the solder mask.
The present invention is directed to an improved dry film for forming a solder mask which eliminates some of the problems of both liquid application and conventional dry films. The present invention is also directed to a method of using a dry film to apply a photoimageable layer, which may be one which forms a solder mask or may be a primary photoresist, to a surface, particularly an uneven surface. Dry film carrying a primary imaging resist may be applied by the method of the present invention for forming excellent conformation for tab plating operations.